The present invention relates to an improved semicontinuous process for the preparation of 4,4xe2x80x2-dihydroxydiphenyl sulfone, comprising the following steps:
(a) reaction of phenol with a sulfonating agent,
(b) suspension of the resulting crude product in water heated to at least 40xc2x0 C., which is free of inert organic solvents and can contain residual amounts of unreacted phenol, and filtration of the product, and
(c) recycling of the resulting educt-containing and/or product-containing waste streams into the preparative process.
4,4xe2x80x2-Dihydroxydiphenyl sulfone (xe2x80x9c4,4xe2x80x2-bisphenol Sxe2x80x9d) is of great economic value inter alia as an auxiliary substance for electroplating, as a raw material for the manufacture of condensation resins, for example to be used as leather tanning agents, and for the manufacture of fibers and especially plastics such as polyether sulfones. As the properties of polymers prepared from 4,4xe2x80x2-dihydroxydiphenyl sulfone are highly dependent on the degree of purity and the isomer ratio of the monomers used, it is desirable to have selective processes for the synthesis of 4,4xe2x80x2-dihydroxydiphenyl sulfone. It is also desirable, for economic and ecological reasons, to have synthetic processes with minimal waste streams or none at all.
Synthetic processes which make use of combustible inert organic solvents, such as methanol, ethanol, o-dichlorobenzene or toluene, in the isolation of 4,4xe2x80x2-dihydroxydiphenyl sulfone from the reaction mixture, for example the processes described in JP-A 50/106 937, CN-A 87/100 796, EP-A 220 004 or WO 92/02493, are likewise unacceptable for ecological and safety reasons.
U.S. Pat. No. 3,297,766 has disclosed a semicontinuous process for the preparation of 4,4xe2x80x2-dihydroxydiphenyl sulfone wherein sulfuric acid is reacted with phenol at 180-188xc2x0 C. with continuous distillation of the water of reaction, the reaction mixture is then cooled to 55-75xc2x0 C. and the solid which has precipitated out is filtered off and washed with water to give a mixture of 4,4xe2x80x2-and 2,4xe2x80x2-dihydroxydiphenyl sulfone. The waste streams from the filtration and rinsing, and the phenol discharged during distillation of the water of reaction, are fed back into the next cycle of the preparative process.
JP-A 50/106 936 describes the preparation of 4,4xe2x80x2-dihydroxydiphenyl sulfone by reacting phenol with a sulfonating agent, such as sulfuric acid, at 180-200xc2x0 C. and crystallizing out and separating off the 4,4xe2x80x2-dihydroxydiphenyl sulfone. The latter is isolated by dissolving or dispersing the crude reaction product in 3 to 8% by weight aqueous phenol solution and crystallizing out and separating off the 4,4xe2x80x2-dihydroxydiphenyl sulfone with or without cooling. If there is still a relatively large amount of unreacted phenol in the reaction product, it is only possible to work with the addition of pure water. 1 to 5 parts by weight of aqueous phenol solution are used per part by weight of reaction product. The solution separated off, which contains phenol and water, can be re-used in a subsequent reaction cycle. According to the Example in JP-A 50/106 936, treatment of the reaction product with 8% by weight aqueous phenol solution at 75xc2x0 C., cooling to 40xc2x0 C. and centrifugation of the 4,4xe2x80x2-dihydroxydiphenyl sulfone which has precipitated out gives an isomeric purity of 98.7%.
Although the process of JP-A 50/106 936 provides the desired product in high isomeric purity, it has the disadvantage of working with large amounts of aqueous treatment solution for isolation from the reaction mixture. These amounts on the one hand require sizeable and hence expensive apparatuses and on the other hand reduce the yield. Furthermore, the presence of large amounts of phenol in the isolation of the product has the disadvantage that the phenol can easily remain stuck to the product as an impurity by forming adduct complexes therewith.
It is known that sulfonation reactions of phenol with sulfuric acid involve reversible equilibria which, with further phenol, proceed via phenol-2-sulfonic acid and phenol-4-sulfonic acid to give 2,4xe2x80x2- and 4,4xe2x80x2-dihydroxydiphenyl sulfone. The isomerization equilibrium which exists between 2,4xe2x80x2- and 4,4xe2x80x2-dihydroxydiphenyl sulfone favors the thermodynamically more stable 4,4xe2x80x2 isomer. However, the isomerization is kinetically inhibited and a catalyst, for example phenol-4-sulfonic acid, is usually necessary for the equilibrium to be reached within a period acceptable in terms of process engineering. The isomerization equilibrium can also be shifted in favor of the 4,4xe2x80x2 isomer by selectively withdrawing it. Furthermore, if the phenol is present in less than the stoichiometric amount, trimers, such as 2,4xe2x80x2,4xe2x80x3-trihydroxytriphenyl disulfone, and higher phenolsulfonic acid oligomers can be formed.
It is an object of the present invention to provide a preparative process for 4,4xe2x80x2-dihydroxydiphenyl sulfone which is free of inert organic solvents in the steps involving the reaction and the isolation of the product from the reaction mixture, in which the waste streams can be recycled into the preparative process, which yields the desired product in high isomeric purity and high yield, even without downstream purification operations, and which, in the isolation of the product from the reaction mixture, involves the smallest possible amounts of treatment solution, the latter being substantially free of unreacted phenol.
We have found that this object is achieved by the semicontinuous process for the preparation of 4,4xe2x80x2-dihydroxydiphenyl sulfone defined at the outset, wherein, in carrying out step (b), the crude product and water are used in a weight ratio of 85:15 to 55:45, preferably 75:25 to 60:40.
The hot water used to suspend the reaction mixture is free of inert organic solvents, especially water-miscible solvents such as methanol, ethanol, isopropanol or acetone, or, if appropriate, halogenated hydrocarbons such as toluene, chlorobenzene or dichlorobenzenes. The residual amounts of unreacted phenol which can be dissolved out of the reaction mixture in the suspension step, or which may already be present to a small extent in the water used, if the latter originates from the previous cycle of the preparative process and has been produced by distillation of the mother liquor from step (b) of said cycle, can be up to a maximum of 2.5% by weight, particularly 0.05 to 1.5% by weight and very particularly 0.1 to 1.0% by weight, based in each case on the amount of water used.
In one preferred embodiment, the water added for treating the reaction mixture in step (b) is at a temperature of 80xc2x0 C. to 100xc2x0 C. Particularly good results are achieved with water at 90xc2x0 C. to 100xc2x0 C. In principle, it is also possible to work at temperatures above 100xc2x0 C., for example at 100xc2x0 C. to 130xc2x0 C., if the treatment with water is carried out in a closed apparatus under superatmospheric pressure. The treatment water can also be added in the form of steam. The duration of the treatment with water is not critical and as a rule is 5 minutes to 5 hours. The treatment with water is normally accompanied by thorough mechanical mixing, for example by stirring.
In another preferred embodiment, the suspension of the crude product in water and the filtration of the product, in step (b), are carried out at approximately the same temperature. Approximately the same temperature conventionally means working in a temperature range of approx. 10xc2x0 C., especially approx. 5xc2x0 C. This embodiment avoids cooling of the suspension, which would result in the crystallization of dissolved constituents.
The suspension (xe2x80x9cslurryingxe2x80x9d) and the filtration can be carried out by the techniques and with the apparatuses conventionally used for this purpose, for example with stirred tanks, mixers or kneaders for the suspension, and with suction filters or centrifuges for the filtration.
In another preferred embodiment, the product filtered off in step (b) is rinsed with water heated to at least 40xc2x0 C., preferably 80xc2x0 C. to 100xc2x0 C. and especially 90xc2x0 C. to 100xc2x0 C. The amount of water used for rinsing should be as small as possible and is normally at most 100%, preferably at most 85%, of the water used to suspend the reaction mixture.
Step (b) is very particularly preferably implemented in such a way that the suspension of the crude product in water and the filtration of the product are carried out at approximately the same temperature in the range 80xc2x0 C. to 100xc2x0 C., especially 90xc2x0 C. to 100xc2x0 C., and the filtered product is rinsed with at most 100%, especially at most 85%, of the water used to suspend the reaction mixture, which is also at approximately the same temperature as in the suspension and filtration operations.
One advantage of the process according to the invention is that the product prepared according to the abovementioned measures in step (b) normally already has an average purity of  greater than 96%, especially  greater than 97.5%, and an average isomeric purity of  greater than 98%, especially  greater than 98.4%. This high purity and isomeric purity are achieved by the fact that, in contrast to all the processes known hitherto, the already very pure crude crystals of 4,4xe2x80x2-dihydroxydiphenyl sulfone formed during the reaction in step (a) are not destroyed, but are freed by the hot treatment water of the 2,4xe2x80x2-dihydroxydiphenyl sulfone, which is more soluble in the mother liquor and accumulates therein. Although the high selectivity of the procedure causes a loss of yield due to the solubility of 4,4xe2x80x2-dihydroxydiphenyl sulfone, this loss is ultimately compensated by the recycling of the waste streams according to step (c) and the semicontinuous procedure.
Another advantage of the process according to the invention is that, after distillation of the water, the mother liquor from step (b), containing mainly 2,4xe2x80x2-dihydroxydiphenyl sulfone, is re-used in step (a) at the beginning of the next cycle of the preparative process with skilful utilization of the chemical equilibrium. The residue of the mother liquor is thereby converted to phenolsulfonic acid. The water distilled off can likewise be re-used in step (b) of the next cycle.
The process according to the invention is also distinguished by a high overall yield generally exceeding 90%, especially of 92% or more, based in each case on the phenol used.
The 4,4xe2x80x2-dihydroxydiphenyl sulfone prepared by the process according to the invention is also distinguished by its good crystallization behavior, i.e. the crystals obtained are large and well formed and have a low dust content, so they can be handled and metered easily.
The sulfonating agent used in step (a) is preferably concentrated sulfuric acid, oleum (xe2x80x9cfumingxe2x80x9d sulfuric acid) or sulfur trioxide (xe2x80x9csulfuric anhydridexe2x80x9d). It is particularly preferable to use concentrated sulfuric acid with a content of approx. 70 to 100% by weight, for example the approx. 96% by weight sulfuric acid available commercially.
Step (a) of the process according to the invention can be carried out as described below.
The phenol and the sulfonating agent can be combined and then brought to the reaction temperature, although the phenol can also be metered in portions or continuously throughout the reaction, especially after the heating phase.
The reaction temperature is conventionally 140xc2x0 C. to 230xc2x0 C., particularly 150xc2x0 C. to 200xc2x0 C. and very particularly 160xc2x0 C. to 175xc2x0 C. If the reaction temperature is below 165xc2x0 C., it is advisable to work under reduced pressure to ensure that the phenol boils.
In the overall reaction in step (a), the molar ratio of phenol to sulfonating agent, for example sulfuric acid, is normally 1.6:1 to 25:1, preferably 1.8:1 to 10:1 and particularly preferably 1.9:1 to 2.5:1.
The water formed in the sulfonation of phenol with sulfuric acid or phenolsulfonic acid can be continuously distilled azeotropically out of the reaction by means of excess phenol present in the reaction mixture, and the phenol can be recycled into the reactor after separation of the water phase. After the azeotropic distillation (xe2x80x9cextractionxe2x80x9d) of usually at least 50% but preferably 90 to 100% of the theoretical amount of water formed, no more phenol is generally recycled into the reaction, but the phenol is gradually distilled off, expediently at 160xc2x0 C. to 200xc2x0 C. The unreacted phenol is re-used in the next cycle of the preparative process.
In one preferred embodiment, firstly phenol is extensively reacted with the sulfonating agent in step (a) to give phenolsulfonic acid, any water of reaction thereby formed is removed by distillation, the phenolsulfonic acid is then reacted with more phenol to give dihydroxydiphenyl sulfone and the water of reaction thereby formed is again removed by distillation.
This is generally followed by an isomerization phase of the reaction mixture at a temperature conventionally of 140xc2x0 C. to 180xc2x0 C., especially 150xc2x0 C. to 170xc2x0 C., and for an isomerization time normally of 0.1 to 10 hours, especially 1 to 4 hours. During this isomerization phase, the equilibrium shifts in favor of 4,4xe2x80x2-dihydroxydiphenyl sulfone and the amount of 2,4xe2x80x2-dihydroxydiphenyl sulfone decreases. The isomerization is normally catalyzed by small amounts of phenolsulfonic acid, which conventionally is still present in small amounts in the reaction mixture, especially when working with a slight stoichiometric excess of sulfonating agent.
In one preferred embodiment, excess phenol is virtually completely removed by distillation after the reaction of step (a) but generally before an isomerization phase.
Before the hot treatment water is added according to step (b), the reaction mixture is conventionally cooled to temperatures below 130xc2x0 C.
The recycling of the resulting educt-containing and/or product-containing waste streams into the preparative process according to step (c), i.e. after the distillation of water into the next cycle of the preparative process, preferably takes place in a closed system, i.e. completely, based on the phenol used, the phenolsulfonic acid intermediate and the dihydroxydiphenyl sulfones. The recycling of phenol distilled off azeotropically with water in step (a) and from the mother liquor in step (b), which remains dissolved in the condensate and cannot be obtained as a separate phase by conventional phase separation, usually turns out to be uneconomicxe2x80x94especially since these amounts of phenol are negligibly smallxe2x80x94and can therefore be omitted.
Semicontinuous procedure is to be understood as meaning the multiple repetition (xe2x80x9ccyclic procedurexe2x80x9d) of non-continuous process steps, with the recycling of process streams. The number of cycles for which the semicontinuous process according to the invention can run is basically unlimited. Conventionally, after the third or fourth cycle, the equilibrium has reached a quasi-stationary state as regards the characteristics of the recycled waste streams and the purity and conversion data of the overall reaction. Remarkably, no impurities or by-products accumulate, as far as can be determined after approx. 10 cycles.
The 4,4xe2x80x2-dihydroxydiphenyl sulfone obtained in step (b) can be further purified by conventional recrystallization. This is preferably effected from a water/acetone mixture generally having a volume ratio of 50:50 to 97:3, especially 80:20 to 95:5. Instead of acetone, however, it is also possible to use other water-soluble or water-miscible organic solvents, e.g. aliphatic alcohols having from 1 to 3 C atoms, such as methanol, ethanol, n-propanol or isopropanol, dimethyl sulfoxide, dimethylformamide, dioxane, ethylene glycol, propylene glycol or higher ethylene or propylene glycols having up to 3 glycol units, in conventional mixing ratios with water. The concomitant use of activated charcoal in the recrystallization is also often advantageous, the resulting pure product generally having an isomeric purity of at least 99.7% in respect of 4,4xe2x80x2-dihydroxydiphenyl sulfone.
The following Example will illustrate the present invention without implying a limitation.